Horizontal drilling through casing window

ABSTRACT

A chemically removable casing section is spotted in a casing string opposite a formation through which a horizontal borehole is to be drilled. The casing section is selectively dissolved by a chemical solution, and a horizontal borehole is drilled into the formation through the window in the casing string.

BACKGROUND OF THE INVENTION

This invention relates to drilling of horizontal boreholes throughsubterranean formations, and more particularly to drilling of horizontalboreholes through formations traversed by a cased well.

There is a renewed interest in horizontal drainholes in the petroleumindustry. Used in the proper reservoirs, horizontal drainholes canprovide an economic alternative to vertical wells with or withoutstimulation. Since the horizontal well concept was introduced in the1930's, drilling, completion, and production technology havesignificantly improved. The applications for drainhole technology havegenerally fallen into five categories. The first is in high permeabilityformations. A second application is for reservoirs which haveheterogeneous permeability distribution, such as naturally fracturedformations. The third is for increased production rates where water orgas coning is a problem. A fourth is to improve the areal sweepefficiency of secondary and tertiary recovery projects. The finalapplication is in thermally enhanced recovery of heavy oil, such assteam floods and fire floods. Horizontal wells can provide higherproduction rates, improved recoveries, better areal sweep, and canreduce the need for additional wells.

SUMMARY OF THE INVENTION

According to the present invention, a method for drilling horizontalboreholes through a formation traversed by a cased well is provided. Themethod comprises providing a casing section adjacent the formation whichis readily soluble in a selected chemical solution, contacting thecasing section with the selected chemical solution to dissolve thecasing section and provide a "window" to the formation, and thendrilling at least one generally horizontal borehole through the windowinto the formation.

THE DRAWINGS

FIG. 1 is a cross sectional view of a cased and cemented wellbore havinga soluble casing section opposite a selected formation.

FIG. 2 is a cross sectional view illustrating the technique for removingthe soluble casing section.

FIG. 3 is a cross sectional view illustrating a production string forproducing fluid produced through a horizontal borehole through theselected formation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

There are several situations in which a generally horizontal boreholeprovides improved results compared to a generally vertical boreholeextending through a formation.

Applications for horizontal borehole technology have generally falleninto five categories. The first is in high permeability formations. Inhigh permeability formations, the pressure drop due to radial flow inthe formation may be as low or lower than the pressure drop due to flowdown the fracture. Consequently, a horizontal well will produce at ahigher rate than a hydraulic fracture of the same length. The secondcategory is reservoirs which have heterogeneous permeabilitydistribution. The horizontal wellbore may enhance contributions by thebest permeability if properly oriented. The third category is forincreased production rates where water or gas coning is a problem. Ahorizontal wellbore in the oil zone may make it possible to obtainhigher production rates without coning. The fourth is for improvement ofareal sweep efficiencies in secondary and tertiary operations.Horizontal wells can be oriented in line drives and five spot or otherpatterns to provide very efficient sweep patterns. The horizontal wellsimprove total flood recovery and improve injection rates. With higherinjection rates which are evenly distributed in the formation, higherproduction rates can be sustained. The final category is for thermallyenhanced recovery projects for heavy oil such as steam floods and firefloods. Multiple horizontal wellbores provide additional surface area inthe reservoir for heat transfer and, therefore, make the heating cyclemore effective.

In cases where there are multiple producing or injection formations, orwhere it is desirable to have a cased section below the formationthrough which the horizontal borehole is to drilled, such as to providea protected area for pumping equipment, a well traversing a producing orinjection formation is first drilled and cased, and then a window in thecasing is created and a generally horizontal borehole is drilled throughthe window in accordance with the invention.

In the past, the window in the casing string has been created by use ofa section mill to cut away the casing opposite the formation ofinterest. This usually involves use of more than one section mill, orperiodic redressing of a section mill, and can take several days.

The preferred embodiment of the present invention involves spotting achemically removable section of casing opposite the formation ofinterest, and then contacting the removable casing section with achemical solution which preferentially dissolves the removable section,followed by drilling a generally horizontal borehole through the thuscreated window in the casing string.

The removable casing section is formed of material which is strongenough to support the casing below the removable section, and yetreadily soluble in a selected chemical solution which does not readilydissolve the remainder of the casing. Preferably, the removable sectionis formed of aluminum or magnesium, or an alloy of aluminum ormagnesium. The selected chemical solution may be an acid or an alkali.To minimize damage to the rest of the casing, a caustic solution ispreferred. A strong solution of alkali metal or ammonium hydroxide withalkali metal or ammonium nitrate is particularly effective in dissolvingaluminum or magnesium.

Raferring to FIG. 1, a wellbore 10 traverses formation 12 and is casedwith casing string 14 extending below formation 12. Casing string 14 iscemented with cement 16, and a section 18 of casing string 14 is spottedadjacent formation 12. Casing section 18 is formed of a material whichis soluble in a selected chemical solution which can dissolve casingsection 18 but not the rest of the casing.

As seen in FIG. 2, casing section 18 may be isolated with upper packer20 and in some cases with a lower packer 22 prior to being contactedwith the selected chemical solution which is injected through injectiontubing 24.

The chemical solution is circulated through injection tubing 24 and upthrough return tubing 26 until the casing section has substantiallydissolved. In many cases it will be desirable to run a section mill orreaming tool down the well to positively clean out any undissolvedportions of casing section 18 and to remove the cement adjacent theformation as shown in FIG. 3, and to circulate out cuttings and anyundissolvad pieces of casing section 18.

As also shown in FIG. 3, a generally horizontal borehole 27 is thendrilled through the window in the casing string, and appropriatecompletion procedures are carried out depending on whether the well isto be a producing well or an injection well, and whether additionalproducing or injection formations (not shown) are to be operatedsimultaneously with formation 12. A pump 28 is positioned adjacent or lobelow formation 12, and is used to pump fluid produced from formation 12up through tubing 26. If the well is to be used as an injection well,upper packer 20 would be left in place to facilitate pumping fluid intoformation 12. Lower packer 22 or some other means, depending on whatuses, if any, are made of the portion of the well below formation 12, isused to also facilitate injection into formation 12. Multiple horizontalboreholes (not shown) could be drilled into formation 12 if desired.

Drilling of generally horizontal boreholes into a formation from avertical well traversing the formation can be carried out by any numberof procedures utilizing various types of hardware depending on the turnradius of the vertical to horizontal section, the length of horizontalborehole to be drilled, and other factors. The present invention makesit possible to carry out the horizontal drilling through a chemicallyremoved window in a casing string extending below the formation throughwhich the horizontal borehole is to be drilled.

I claim:
 1. In a process for drilling and completing a well traversing aformation wherein casing is set in the well to a point below saidformation, wherein a section of casing that is more soluble in aselected chemical than the remainder of said casing is located in saidcasing adjacent said formation, and wherein said more soluble section ofcasing is contacted with said selected chemical solution in an amountand for a time sufficient to substantially dissolve said more solublecasing section, the improvement comprising:(a) drilling at least onegenerally horizontal borehole segment into said formation through theopening created by dissolving said more soluble casing section.
 2. Theprocess of claim 1 wherein a pump is positioned in said well to pumpproduced fluids from said generally horizontal borehole segment to thesurface.
 3. The process of claim 1 wherein sealing means are positionedin said well above and below said formation, and fluid is injected intosaid formation through said generally horizontal borehole segment. 4.The process of claim 1 wherein, prior to drilling said generallyhorizontal borehole segment, a section mill is run in said well toremove any undissolved portion of said substantially dissolved casingsection, and fluid is circulated through said well to remove solidparticles therefrom.
 5. The process of claim 1 wherein said more solublecasing section is made from material selected from the group consistingof aluminum, magnesium, and alloys of aluminum or magnesium.
 6. Theprocess of claim 5 wherein said selected chemical solution comprises acaustic solution.
 7. The process of claim 6 wherein said selectedchemical solution is a solution of sodium hydroxide and a dissolvednitrate salt.